1. Field of the Invention
The present invention relates to the field of underground storage tanks and, more specifically, to an apparatus and method for providing added leakage protection and leakage detection capabilities for underground storage tanks.
2. Description of the Related Art
In recent years, concerns have been raised over the environmentally hazardous leakage of petroleum products and other materials from insecure underground storage tanks. Several states have adopted laws requiring new underground tanks to provide some form of secondary containment to prevent leakage from single wall tanks. Some state codes also require that such new tanks incorporate a leak detection means capable of detecting leakage of the stored product from the primary containment means into the secondary containment means or leakage of ground water or other liquids from outside the secondary containment means through the secondary container to provide a warning of any leakage before leakage through both first tank, creating in essence, a double walled tank with a leakage collection space therebetween. Currently, the most common manner in which leakage is being prevented is through the use of a steel secondary containment vessel. However, this configuration is not as effective in reducing leakage because the secondary steel shell is still susceptible to corrosion in the event of coating damage. Furthermore, use of a steel on steel configuration is expensive. Other types of such double walled tanks are known in the prior art.
U.S. Pat. Nos. 4,523,454 and 4,607,522, both issued to B. R. Sharp disclose underground storage tank systems for liquids having secondary containment capability and leakage detection capability utilizing detecting fluid. Borh systems disclosed utilize a flexible jacket to encase a rigid inner tank.
U.S. Pat. No. 4,561,292 issued to Rignale, discloses a tank having two fiberglass walls, the second fiberglass wall being formed by applying a fiberglass layer over a grid of plastic mats separated by fiberglass ribs.
U.S. Pat. No. 4,537,328, issued to K. M. Keesee et. al., discloses a tank with a steel inner tank and a fiberglass outer tank, the fiberglass outer tank being formed by applying fiberglass sheets over a grid of semicircular PVC pipe segments attached to the surface of the inner steel tank.
U.S. Pat. No. 4,744,137, issued to P. T. Palazzo, discloses a method for making a double wall storage tank. In the method disclosed by Palazzo a mold member is formed having a cylindrical configuration generally similar to but somewhat larger in diameter than the configuration of the inner tank. The sidewall surface of the mold has a plurality of depressions each extending generally radially inwardly. A moldable, hardenable material is then applied over the sidewall surface. It is desired for the moldable, hardenable material to conform to the mold sidewall surface and depressions therein. A cylindrical sheath is thereby formed during the molding process with a plurality of projections extending generally radially inwardly. When the cylindrical sheath has cured the sheath is removed from the mold by forming an opening or slit through the sheath, the opening extending the full axial length of the sheath. This cylindrical sheath is then introduced over the inner tank, the radially inward projections of the sheath spacing the sheath from the outer surface of the inner tank to permit passage of liquid therebetween. Migration of fluid is important in order to allow the detection of any leakage of fluid through either the primary tank or the secondary tank into the space between the inner tank and the cylindrical sheath.
The Palazzo method works well when carefully carried out. However, the resulting tank must be handled appropriately, as the fiberglass sheath is only supported at each projection. Thus, the tank must be carefully handled in shipment and when placed on the ground, or cracking may occur due to the high stresses in the outer sheath at the projections. This problem is also aggravated by any failure of the depressions in the mold to fill during the molding of the outer sheath, requiring care both in cleaning the mold, particularly the depressions therein before starting, and in assuring that the depressions properly fill during the molding process, as merely wrapping the mold with resin saturated fiberglass will not assure that the depressions properly fill, particularly with depressions of the size and shape of those disclosed in U.S. Pat. No. 4,744,137.